1. Field of the Invention
The invention relates to an electrode-electrolyte unit for a fuel cell, comprising a proton-conducting electrolyte which on one side is provided with a catalytically active anode and on the opposite side is provided with a catalytically active cathode and which operates using a fuel which is deprotonated at the anode. The fuel used can e.g. be hydrogen or methanol. Potentially suitable electrolytes include membranes or other solid electrolytes, e.g. made of ceramic material, or liquid electrolytes.
2. Description of Related Art
Fuel cells are systems which convert chemical energy into electrical energy. The central electrochemical functional element of a fuel cell is the electrode-electrolyte unit. Such an electrode-electrolyte unit comprising a ceramic solid electrolyte is disclosed e.g. by DE 40 33 286 A1. Further proton-conducting solid electrolytes in the form of oxides or fluorides are proposed in DE 39 29 730 C2=EP 0 417 464 A1.
Membrane fuel cells have an ion-conducting membrane which is disposed between two catalytically active electrodes, the anode and the cathode. The membrane used is a polymer material, for example. The anode material used is preferably platinum or a platinum-ruthenium alloy, the cathode material used preferably being platinum. The anode material and cathode material are either deposited on the membrane by a wet chemical process, or it is present as a powder and is hot-pressed with the membrane.
DE-C 42 41 150 describes methods according to which such membrane-electrode units can be fabricated.
In a fuel cell which, as stated at the outset, is operated directly using methanol, so-called direct-methanol fuel cells, or using another fuel which is deprotonated at the anode of the membrane-electrode unit, the protons permeate the electrolyte layer and react at the cathode side with the oxygen supplied there to form water. Fuel cells running on hydrogen work in a similar manner.
A drawback of the known fuel cells is that not only the ions are able to pass through the electrolyte, but to some extent also the hydrate shells of the hydrogen ions or part of the fuel. In the case of methanol-consuming fuel cells, the electrolyte is permeable to methanol molecules.
The drawback is, firstly, that the methanol poisons the cathode, leading to a reduced cell voltage, and secondly that the oxidizable fraction of the methanol at the anode is reduced, thereby reducing the fuel utilization factor of the fuel cell.
In hydrogen fuel cells the entrainment of water causes the anode to dry out, leading to reduced output. Consequently, additional humidification of the hydrogen is required.
Previous approaches to solving the problem of methanol diffusion in the case of direct-methanol fuel cells were directed, inter alia, at improving the anode kinetics, e.g. via appropriate anode activity, thereby causing all of the methanol to react at the anode, so that a low methanol concentration is established at the phase boundary anode/electrolyte. This is meant to ensure a reduction in the amount of methanol which penetrates and permeates the electrolyte layer. No anode structures, however, have been disclosed hitherto which would be adequately able to prevent the diffusion of methanol under all operating conditions.